Vehicle wheels are generally mounted to the chassis of the vehicle by an axle and a suitable lubricant such as grease or lubrication oil is commonly applied between the hub of the wheel and the spindle of the axle to allow the wheel to rotate about the spindle when the vehicle is in motion. Oil hub caps are commonly employed in large commercial vehicles such as large commercial trucks and trailers. The oil hub cap within the interior contains lubricating oil to provide the lubrication between the wheel hub and the spindle. It is common to provide for a visual inspection means at the front of the oil hub cap to allow a user to visually check the amount of the lubricant in the cap. In prior art embodiments, the casing of the oil hub cap is covered by a clear window and the surface of the window is etched with two concentric circles indicating a full level of lubricant within the oil hub cap or that the level of lubricant must be replenished. Examples of such oil hub caps are shown in U.S. Pat. Nos. 5,752,746; 6,273,519 and 6,447,072 among others.
While such prior art arrangements do allow for visual inspection, it can be difficult in some circumstances to clearly view the lubricant level within the oil hub cap. This is particularly the case if the lubricant level is being checked at inadequate light level where it may be difficult to be able to clearly see the levels even with the use of a flashlight as the flashlight may either reflect off the surface of the window obscuring the fluid level or if the fluid has been recently changed, the colour being a lighter colour may be difficult to observe through the window.
There thus remains a need to allow for easy determination of the fluid levels within an oil hub cap under any conditions.